This invention relates to controllable power sources and is particularly suitable for controlling the amount of electric power which is applied to a machine from a low voltage electric source, such as a battery. It is generally undesirable to drive a high power machine, such as a rotatable motor directly from a low voltage battery, since the current required is extremely large and this results in consequential excessive electrical losses, i.e. the efficiency with which power is transferred from the battery to the motor is relatively low, and can be particularly low when the motor is operating at maximum power. In principle it is more efficient to convert the low voltage which is available from the battery to a high voltage which is then applied as required to a motor which is designed to operate at the high voltage. It can be shown that such a system is capable of higher overall efficiency, but even so it is difficult and sometimes impracticable to design a controllable power source which is capable of providing maximum power for all possible combinations of current and voltage demands.